[feature] Allow loading TLS certs from disk (#1586)

Currently, GtS only supports using the built-in LE client directly for
TLS. However, admins may still want to use GtS directly (so without a
reverse proxy) but with certificates provided through some other
mechanism. They may have some centralised way of provisioning these
things themselves, or simply prefer to use LE but with a different
challenge like DNS-01 which is not supported by autocert.

This adds support for loading a public/private keypair from disk instead
of using LE and reconfigures the server to use a TLS listener if we
succeed in doing so.

Additionally, being able to load TLS keypair from disk opens up the path
to using a custom CA for testing purposes avoinding the need for a
constellation of containers and something like Pebble or Step CA to
provide LE APIs.

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